Have been to both- my opinion:
If your a beginner/early intermediate going with wife/family then id recommend Pamporovo. Nicer, cleaner, smaller resort.
If its a boys holiday then choose Borovets regardless of everyones ski ability (it suits all levels). theres plenty of `fun` bars to relax in after a day on the slopes. Lap dancing bars in abundance although having been to them all `Boni Bar` is without question THE DADDY!.